The present invention relates to a composition and method for enhancing the recovery of petroleum from an oil-bearing formation. In the recovery of oil from reservoirs, the use of primary production techniques (i.e., the use of only the initial formation energy to recover the crude oil) followed by the secondary technique of waterflooding, recovers only a portion of the original oil present in the formation. Moreover, the use of certain enhanced oil recovery (EOR) techniques is also known in the art. One such technique is a gas-drive method wherein a gas, such as carbon dioxide, is used to facilitate the recovery of oil.
Surfactants are well known for their use as foaming agents in enhanced oil recovery techniques. See, for example, U.S. Pat. No. 4,380,266 to Wellington, U.S. Pat. No. 4,860,828 to Oswald et al. and U.S. Pat. No. 5,502,538 to Wellington et al. One purpose of foam is to divert the flow of the drive-gas into that portion of the formation containing high oil saturation.
Useful surfactants for enhanced oil recovery are alpha-olefin sulfonate surfactants. Generally, an “alpha-olefin sulfonate” is a mixture containing a hydroxyalkane monosulfonate, an alkene monosulfonate and a small amount of an alkene disulfonate. Each of these components further contains several types of structural isomers. The mixture is recognized as an alpha-olefin sulfonate (AOS) in the surfactant industry.
While alpha-olefin surfactants are largely known as “good foamers”, they are also known to suffer from numerous stability issues. For example, solubility is limited when employing alpha-olefin surfactants having longer chain lengths, i.e., greater than about C10, by the amount of salt in the injection water or formation brine. Another drawback relates to transportation, use, and/or storage of aqueous alpha-olefin sulfonates at low temperatures; reduced solubility at low temperatures often results in alpha-olefin sulfonate solutions becoming turbid.
Since each of the components of the alpha-olefin sulfonate has a different crystallization point, the alpha-olefin sulfonate causes turbidity where the component having the highest crystallization point is first allowed to precipitate as crystals at low temperatures. Once it causes white turbidity, the alpha-olefin sulfonate is unlikely to be easily returned again to a transparent state. Phase separation can result in high viscosity and/or gelling problems.
It is well known that adding a hydrotrope (a compound possessing the ability to increase the water-solubility of an organic compound) to a detergent may improve its stability. For example, alpha-olefin compositions comprising alkali metal salts, ammonium salts, or organic base salts of an alkyl- or alkenyl polyglycol ether carboxylic acid to an alpha-olefin sulfonate having 8 to 20 carbon atoms are disclosed in GB 1,225,218. Alpha-olefin compositions comprising substituted fatty acid amido component and a sulfonated hydrotrope component are taught in U.S. Pat. No. 3,852,221. U.S. Pat. No. 4,367,169 discloses alpha-olefin compositions comprising a pyridine salt. While it is well know that adding such hydrotropes to aqueous surfactant compositions may depress the freezing point of the water, they may not necessarily improve the stability of the surfactant compositions at low temperatures to a sufficient extent.
Accordingly, it is desirable to provide aqueous surfactant compositions, especially aqueous AOS compositions, which have good stability at low temperatures; for example, at −5° C. or lower in winter in accordance with storage and use conditions.